Coinfection with Borrelia burgdorferi and Babesia microti is a clinically documented, emerging public health concern in the Northeastern United States. However, to date, it still remains unclear whether or not concurrent infection with these pathogens leads to greater disease severity in humans. To explore whether concurrent infection with these two emerging human pathogens leads to greater disease severity, we have proposed to develop an animal model to quantify the polymicrobial burden as well as several clinical parameters during single and concurrent experimental infection in mice of different predisposing conditions and genetic backgrounds. In addition to evaluating disease severity during single and concurrent Borrelia and Babesia infection in mice, we will also monitor the differential genetic expression of B. burgdorferi in response to B. microti in feeding larval ticks and infected mouse tissues utilizing whole genome DNA arrays. To further define the innate immunological response of the vertebrate host during polymicrobial infection, genomic expression analysis will be carried out in murine and human monocytic cells stimulated with live B. burgdorferi and B. microti alone and concurrently. [unreadable] [unreadable]